Method for producing an aqueous industrial sodium chloride solution

ABSTRACT

Process for the manufacture of an industrial aqueous solution of sodium chloride, according to which an aqueous mixture (12) containing an aqueous solution of sodium chloride and of polyvalent metals is prepared by introducing into water (10) a solid product (8) collected from the dry-route treatment, with sodium bicarbonate (4), of smokes (2) containing hydrogen chloride, and the said aqueous medium (12) is treated in order to remove the polyvalent metals therefrom.

FIELD OF THE INVENTION

The invention relates to a process for the manufacture of aqueous sodiumchloride solutions which can be employed in industrial processes.

TECHNOLOGY REVIEW

Aqueous sodium chloride solutions find important applications inindustry. This is the case in particular in the industry for themanufacture of sodium carbonate by the ammonia-soda process, and in theprocesses for electrolytic manufacture of chlorine and of aqueous sodiumhydroxide solutions.

These industrial processes generally require aqueous sodium chloridesolutions of high purity, especially in respect of polyvalent metalssuch as, for example, calcium, magnesium, aluminium, iron, lead andzinc. This requirement is particularly strict in the case ofelectrolytic processes using membranes which are selectively permeableto cations, such as perfluorinated polymer membranes containingfunctional groups derived from carboxylic acid (GB-A-1,375,126). Inthese electrolytic processes sodium chloride solutions are generallyrecommended in which the polyvalent cation content, especially ofcalcium cations, does not exceed 0.1 ppm (GB-A-2,005,723).

The aqueous solutions intended for the electrolysis cells are generallyobtained by dissolving rock salt in water (Chlorine, Its manufacture,Properties and Uses--J. S. Sconce--Reinhold Publishing Corporation, NewYork--1962--pages 119 and 123). However, the raw brines obtained bydissolving rock salt in water generally have a polyvalent cation contentwhich is too high to permit their use in the electrolytic processes. Ithas been proposed, for this purpose, to treat them on chelating resinsof the Na type, with the aim of extracting the polyvalent cationstherefrom. (The American Institute of Chemical Engineers, No. 219, vol.78, 1982, pages 46-53 :J. J. Wolff and R. E. Anderson, Ion-exchangepurification of feed brine for chlor-alkali electrolysis cells; the roleof Duolite ES-467).

Furthermore, there is a known process for purifying a smoke containinghydrogen chloride, according to which this smoke is treated with sodiumbicarbonate so as to decompose the hydrogen chloride and to form sodiumchloride (U.S. Pat. No. 4,767,605; SOLVAY & Cie, brochure Tr.895/5c-B-1-1290). According to this known process sodium bicarbonate inpowder form is introduced into the smoke to be purified and a solidresidue containing sodium chloride is separated from the latter. Whenthe smoke which is treated contains heavy metals, as is generally thecase with smokes produced by the incineration of domestic or municipalwaste, the solid sodium chloride residue is contaminated with theseheavy metals, which thus cause difficulties in the storage or dumping ofthis residue.

The invention is aimed at providing a process which makes it possible torecover the solid sodium chloride residue from the known processdescribed above, to manufacture an aqueous sodium chloride solutionwhich can be employed in industrial processes.

SUMMARY OF THE INVENTION

Consequently, the invention relates to a process for the manufacture ofan industrial aqueous solution of sodium chloride, according to which anaqueous medium comprising an aqueous solution of sodium chloride and ofpolyvalent metals is prepared and the said aqueous medium is treated toremove the polyvalent metals therefrom; according to the invention, inorder to prepare the aqueous medium , a solid product collected from thedry-route treatment, with sodium bicarbonate, of a smoke containinghydrogen chloride and polyvalent metals is introduced into water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a plant using a particularembodiment of the invention. FIGS. 2, 3, and 4 are also schematicdiagrams illustrating three other embodiments of the invention. Each ofthese embodiments is described below.

DETAILED DESCRIPTION OF THE INVENTION

In the process according to the invention an industrial aqueous solutionof sodium chloride is intended to denote an aqueous sodium chloridesolution which is intended to be involved in the technical activity ofan industry.

The aqueous medium employed includes an aqueous solution of sodiumchloride and of polyvalent metals. The aqueous solution usually alsoincludes some sodium sulphate resulting from the reaction of sodiumbicarbonate with the sulphur oxide present in the smoke and some sodiumcarbonate corresponding to the excess sodium bicarbonate used in thesmoke treatment. The polyvalent metals of the aqueous medium are metalsof valency higher than 1 and include the heavy metals. These metals maybe present in the form of compounds which are dissolved and/or insolublein the aqueous sodium chloride solution. In accordance with theinvention this aqueous medium is obtained by introducing into water asolid product collected during the dry-route treatment, with sodiumbicarbonate, of a smoke containing hydrogen chloride and polyvalentmetals.

"Dry-route treatment of a smoke with sodium bicarbonate" is intended tomean a treatment in which sodium bicarbonate is introduced in the solidstate into the smoke, in the absence of a liquid, in particular ofwater. In general, the sodium bicarbonate is used in the form of apowder which is injected into a smoke stream moving inside a reactionchamber. In the latter the sodium bicarbonate decomposes the hydrogenchloride present in the smoke, forming sodium chloride. It isadvantageous to employ a sodium bicarbonate powder of uniform particlesize which is as fine as possible, so as to accelerate the decompositionof the hydrogen chloride. As a general rule it is recommended to employa sodium bicarbonate powder whose mean particle diameter is smaller than50 μm. The preferred particle size corresponds to a mean particlediameter not exceeding 25 μm, for example between 5 and 20 μm.

The solid product collected during the treatment of the smoke withsodium bicarbonate contains sodium chloride (resulting from the reactionof hydrogen chloride with sodium bicarbonate) and polyvalent metals inthe metallic or combined state. In the case of a smoke containingsulphur oxides the solid product also contains sodium sulphate(originating from the reaction of sulphur oxides with sodiumbicarbonate). It generally also contains some sodium carbonatecorresponding to the excess of sodium bicarbonate used to react withhydrogen chloride and, where appropriate, sulphur oxides.

The abovementioned aqueous medium is obtained by introducing theabovementioned solid product into water. The quantity of water which isused must be at least sufficient to dissolve all of the sodium chlorideand the other soluble matter in the solid product. Notwithstanding thiscondition, the quantity of water is not critical. However, there is noadvantage in employing an excessive quantity of water. As analternative, a dilute sodium chloride solution can also be employedinstead of water.

The origin of the smoke is not critical, the latter necessarilycontaining hydrogen chloride and polyvalent metals in the metallic orcombined state. The invention applies especially to the smokes producedby the incineration of domestic or municipal waste, which usuallycontain chlorine compounds, metal chlorides and polyvalent metals (VGBKraftwerkstechnik, 69, Part 2, 1989, pages 212-220). Depending on thesource of the waste, the polyvalent metals entrained by the smokesinclude cadmium, mercury, antimony, lead, cobalt, chromium, copper,manganese, vanadium, tin, iron, nickel, calcium, magnesium, aluminiumand zinc, this list not being exhaustive.

In the process according to the invention the removal of the polyvalentmetals from the aqueous medium may be obtained by any appropriate means.

In a preferred embodiment of the process an alkaline aqueous medium isused so as to obtain the polyvalent metals in the form of a precipitateof metal hydroxides, this precipitate is separated from the aqueousmedium and the resulting aqueous liquid is treated on a chelating resin.In this embodiment of the process according to the invention the aqueousliquid has an alkaline pH and can be treated as it is on the chelatingresin. It is advantageous to regulate its pH between 8 and 14. Thealkalinity of the aqueous medium can be produced by the dissolved sodiumcarbonate which it contains and which corresponds to the excess ofsodium bicarbonate used for the smoke treatment. If need be asupplementary addition of an inorganic base may be made, such as, forexample, sodium carbonate or sodium hydroxide.

The chelating resins employed in this form of embodiment of the processaccording to the invention are well known in the art. They include apolymeric backbone onto which are grafted complexing functional groupsincluding interchangeable cations. Examples of polymers which can beemployed for the polymeric backbone include polyolefins (for examplepolyethylene), polymers derived from styrene (for example copolymersderived from styrene and divinylbenzene) and acrylic resins. The resinis generally in the form of granules, in contact with which the aqueousliquid is circulated.

In the form of embodiment of the invention which has just been describeda chelating resin of the H type can be used just as well as one of theNa type. However, resins of the Na type (which are resins in which theinterchangeable cations are sodium cations) are preferred because,everything else being equal, they ensure an optimum extraction of thepolyvalent metal ions, especially of calcium, according to the processdescribed in the document "The American Institute of Chemical Engineers,No. 219, vol. 78, 1982, pages 46-53: J. J. Wolff and R. E. Anderson,Ion-exchange purification of feed brine for chlor-alkali electrolysiscells; the role of Duolite ES-467". At the end of the process the resinmust be regenerated, and this can be done in a manner known per se bytreating it with an aqueous solution of hydrochloric acid; in the caseof a resin of Na type the treatment with the hydrochloric acid solutionis followed by a treatment with an aqueous sodium hydroxide solution.

The preferred chelating resins within the scope of the invention arethose in which the functional groups include a nitrogen ligand. Thelatter may, for example, include compounds derived from amines orimines. Chelating resins which are especially recommended are thoseincluding functional groups derived from organic acids, those includingfunctional groups derived from iminodiacetic acid or aminophosphonicacid being preferred. Such resins are described especially in PatentU.S. Pat. No. 4,002,564 (Diamond Shamrock Corp.) and in PatentApplication EP-A-0,087,934 (Duolite International S.A.). Examples ofresins which can be employed in the process according to the inventionare those marketed under the names Duolite (Rohm & Haas Company) andLewatit (Bayer AG). In the particular case where the aqueous liquidcontains mercury it is advantageous to precede the treatment on theresin containing nitrogen ligands with a treatment on a chelating resincontaining sulphur ligands, in order to bind the mercury thereon.

In a particular alternative form of the preferred embodiment thealkaline aqueous medium is acidified and filtered. In this alternativeform of the invention the acidification can be carried out by additionof an aqueous hydrochloric acid solution to the aqueous medium. Itspurpose is to decompose the sodium carbonate and to dissolve thepolyvalent metals. The filtration is used to remove the insolublematter, especially the fly ash from the smoke.

When the aqueous medium contains SO₄ ²⁻ anions, it may be desirable, insome applications, to prevent these anions from moving into theindustrial aqueous solution of sodium chloride. To this end, accordingto another alternative form of the abovementioned preferred embodimentof the invention, the SO₄ ²⁻ anions are precipitated in the form ofcalcium sulphate with the aid of calcium hydroxide or chloride. In thisalternative form of embodiment of the invention the operation can becarried out by a number of separate routes.

According to a first route the calcium hydroxide or chloride is added tothe aqueous liquid collected after separation of the precipitate ofmetal hydroxides. The calcium sulphate then precipitates in the form ofrelatively uncontaminated gypsum which, as a result, can be easilyrecovered industrially or dumped as such on a tip. Where appropriate, itdoes not require a specialised tip, and this is an economical advantage.In the case where the aqueous medium contains carbonate ions, asufficient quantity of calcium hydroxide or chloride must be used toreact with the sulphate ions and with the carbonate ions of the aqueousliquid, with calcium carbonate then being precipitated before theformation of calcium sulphate.

According to a second operating route the alkaline medium is acidified,for example by addition of hydrochloric acid, and a sufficient quantityof calcium chloride is added to it to form calcium sulphate with thesulphate ions. The objective of the acidification is to decompose thecarbonate ions and to solubilise the polyvalent metals. It is preferablyregulated to impart to the aqueous medium a pH value lower than 6, forexample between 3 and 5. After separation of the calcium sulphateprecipitate (generally in the form of gypsum) the aqueous medium isalkalified (preferably by addition of sodium hydroxide) and is thentreated as described above in order to extract the polyvalent metalstherefrom.

According to a third route the alkaline medium is acidified to beginwith, for example by addition of hydrochloric acid, and is thenalkalified by adding to it calcium hydroxide and, optionally, sodiumhydroxide. The acidification is performed under the same conditions asin the abovementioned second route so as to decompose the carbonate ionsand solubilise the polyvalent metals. The quantity of calcium hydroxidewhich is used subsequently must be at least sufficient to react with allof the sulphate anions of the aqueous medium; furthermore, the overallquantity of calcium hydroxide and sodium hydroxide must be sufficient toproduce an alkaline pH (preferably equal to at least 8) in the aqueousmedium. In this third operating route of the process the calciumsulphate (generally in the form of gypsum) and the metal hydroxidesprecipitate simultaneously, and this constitutes an advantage. It hasbeen observed, in fact, that this coprecipitation facilitates andaccelerates the settling of the precipitate and improves the subsequentseparation of the precipitate and the recovery of the aqueous liquid.

The process according to the invention makes it possible to obtainindustrial aqueous solutions of sodium chloride of very high purity inwhich the polyvalent metal ion content is lower than 1 ppm. Inparticular, it makes it possible to obtain saturated aqueous sodiumchloride solutions which have a calcium weight content lower than 0.1ppm and generally not exceeding 0.05 ppm.

The aqueous sodium chloride solutions obtained by means of the processaccording to the invention consequently find various applications inindustry. They can be used in particular as a raw material for themanufacture of sodium carbonate by the ammonia-soda process (Manufactureof Soda--Te-Pang Hou--Hafner Publishing Company--1969), for theelectrolytic manufacture of chlorine and aqueous sodium hydroxidesolutions, for the electrolytic manufacture of aqueous sodium chloratesolutions and for the manufacture of solid salt.

The invention consequently also relates to the use of the aqueous sodiumchloride solution obtained by means of the process according to theinvention for the manufacture of an aqueous sodium hydroxide solution byelectrolysis or electrodialysis. Processes for electrolytic manufactureof aqueous sodium hydroxide solutions are well known in the art andinclude especially the mercury cathode cell process and the selectivelycation-permeable membrane cell process (Chlorine, Its manufacture,Properties and Uses--J. S. Sconce--Reinhold Publishing Corporation, NewYork--1962 --pages 127 to 199; European Patent EP-B-0,253,430 andBelgian Patent Applications 09000497 (now Belgian Patent 1004126) and09000924 (now Belgian Patent 1003943), all three in the name of Solvay &Cie). Processes for the manufacture of aqueous sodium hydroxidesolutions by electrodialysis are also well known in the art (Patent U.S.Pat. No. 2,829,095 Noguchi Kenkyu-Jo). According to the invention theraw material employed in the electrolysis or electrodialysis is anindustrial aqueous solution of sodium chloride, obtained by means of theprocess in accordance with the invention, described above.

Another subject of the invention is the use of the aqueous sodiumchloride solution obtained by means of the process according to theinvention for the manufacture of sodium chloride crystals. Themanufacture of crystals of sodium chloride of high purity by evaporationof aqueous sodium chloride solutions is well known in the art (SodiumChloride--Dale W. Kaufmann--Reinhold Publishing Corporation, NewYork--1960 --pages 205 to 274; European Patent ApplicationEP-A-0,352,847 --Solvay & Cie). According to the invention an industrialaqueous solution obtained by means of the process in accordance with theinvention, described above, is employed as the aqueous sodium chloridesolution subjected to evaporation.

An additional subject of the invention is the use of the industrialaqueous solution of sodium chloride for the manufacture of sodiumcarbonate by the ammonia-soda technique or by the insoluble aminestechnique. The ammonia-soda technique (also called the Solvay process)is well known and comprises dissolving ammonia gas in an aqueous sodiumchloride solution so as to produce an ammoniacal brine, treatment ofthis ammoniacal brine with a gas containing carbon dioxide so as tocrystallise sodium bicarbonate and the conversion of the latter tosodium carbonate (Te-Pang Hou, Manufacture of Soda with specialreference to the Ammonia Process, 1969, Hafner Publishing Company). Inthe technique using amines an aqueous sodium chloride solution and anorganic solution of a water-insoluble amine are mixed and the resultingmixture is treated with a gas containing carbon dioxide to crystallisesodium bicarbonate, which is collected and which is next converted intosodium carbonate [Patents GB-A-1,082,436 (Kaiser Aluminum & ChemicalCorporation) and FR-A-2,545,079 (Solvay & Cie)]. According to theinvention the aqueous sodium chloride solution employed in these twotechniques is an industrial aqueous solution obtained by means of theprocess in accordance with the invention, described above.

The invention performs, in an original and economical manner, a recoveryof residues originating from smoke purification. It is very speciallysuited to the treatment of smokes originating from plants for theincineration of domestic or municipal waste.

Special features and details of the invention will appear in the courseof the description of a number of embodiments which is to follow, withreference to the attached drawings.

FIG. 1 shows the diagram of a plant using a particular embodiment of theinvention.

FIGS. 2, 3 and 4 are three schemes similar to that of FIG. 1, of plantswhich use, respectively, three other embodiments of the invention.

In these figures the same reference numbers denote identical components.

The plants shown in the figures are designed to purify a smokeoriginating from the incineration of domestic or municipal waste and torecover the effluents collected from the purification of this smoke.This smoke contains hydrogen chloride, sulphur dioxide and polyvalentmetals (for example: cadmium, mercury, antimony, lead, cobalt, chromium,copper, manganese, vanadium, tin, nickel, calcium, magnesium, aluminium,zinc, this list not being exhaustive). It additionally contains fly ash.This smoke is indicated by reference number 2 on the attached drawing.

The plants comprise a tubular reactor 3 fed, in its lower part, with thesmoke 2 emitted by an incinerator for domestic waste 1. Sodiumbicarbonate 4 in the form of an anhydrous powder is injected into thesmoke 2 in the reactor 3. In the reactor 3 the sodium bicarbonate reactswith the hydrogen chloride in the smoke, forming sodium chloride.Furthermore, it reacts with sulphur dioxide, forming sodium sulphate.The smoke 5 extracted from the upper part of the reactor 3 passesthrough a dust separator 6 and is then removed to the stack 7. Since thesmoke 5 is dry, the dust separator 6 may advantageously consist of amechanical separator with filter fabrics (bag filter) whose efficiencyis optimum. Other types of separators can also be employed, such as, forexample, electrostatic filters.

In the dust separator 6 the smoke 5 is freed from the solid particles 8which it contains. The latter include sodium chloride, sodium sulphate,sodium carbonate, polyvalent metals and fly ash.

In the plant shown in FIG. 1 this solid product 8 is introduced into adissolving chamber 9 where it is dispersed in a sufficient quantity ofwater 10 to dissolve all of the water-soluble matter which it contains.The aqueous -medium thus obtained is generally alkaline. Its pH isadjusted to a value of between 8 and 14 by addition of an aqueoussolution 11 of sodium hydroxide or hydrochloric acid, depending onwhether its initial pH is lower or higher than the required value. Thealkaline aqueous medium 12 drawn off from the chamber 9 contains thepolyvalent metals in the form of insoluble metal hydroxides. It istreated on a filter 13 to separate off the insoluble matter 14 (fly ashand polyvalent metal hydroxides). The aqueous liquid 15 collected fromthe filter 13 is an aqueous solution of sodium chloride, sodium sulphateand sodium carbonate. It is contaminated with dissolved polyvalentmetals in a quantity which is generally too high to allow it to be usedin an industrial process. According to the invention it is circulated ina column 16 in contact with a chelating resin containing functionalgroups of the Na type, derived from aminophosphonic acid. Consequently,in column 16 an ion exchange takes place between the sodium cations ofthe resin and the polyvalent cations of the aqueous liquid 15. Anindustrial aqueous solution of sodium chloride 17 is collected from thecolumn 16, and is sufficiently pure to allow it to be employed as suchin an industrial process. The solution 17 containing dissolved sodiumsulphate and carbonate finds use in an industrial plant 18 used for themanufacture of sodium carbonate by the ammonia-soda technique.

In the plant shown in FIG. 2 the alkaline aqueous liquid 15 (aqueoussolution of sodium chloride, sodium sulphate and sodium carbonate,contaminated with dissolved polyvalent metals) is obtained as describedabove with reference to FIG. 1. It is conveyed to a reaction chamber 24where an aqueous calcium chloride solution 25 is added to it in asufficient quantity to react with all of the sodium carbonate andsulphate and to precipitate calcium carbonate and calcium sulphate(gypsum). The aqueous suspension 26 collected from the reaction chamber24 is treated on a filter 27 in order to separate from it the calciumcarbonate and sulphate precipitate 28, which is set aside. The aqueoussodium chloride solution 29 collected from the filter 27 is conveyed tothe column 16 to be treated therein on the chelating resin as describedabove with reference to FIG. 1.

In the plant shown in FIG. 3 a sufficient quantity of hydrochloric acid11 is introduced into the reaction chamber 9 to acidify the aqueousmedium present therein, to decompose the carbonate ions and to dissolvethe polyvalent metals. In addition, an aqueous calcium chloride solution30 is introduced into it in sufficient quantity to react with the sodiumsulphate and to precipitate gypsum.

The acidic aqueous medium 12 collected from the reaction chamber 9 isconveyed to the filter 13, where the insoluble matter 14 (calciumsulphate and fly ash) is separated off. An aqueous solution 31 of sodiumchloride, free from sodium sulphate and carbonate and containingpolyvalent metals in the dissolved state is collected from the filter13. The solution 31 is conveyed to the reaction chamber 24, where it isalkalified to a pH of between 8 and 14 by addition of an aqueous sodiumhydroxide solution 32. The alkalifying results in the precipitation ofthe polyvalent metals in the form of metal hydroxides. An aqueoussuspension 33 is consequently collected from the chamber 24 and istreated successively in a settling chamber 34 and on a filter 35, wherea precipitate 36 of polyvalent metal hydroxides is separated therefromand is set aside. The aqueous liquid 15 collected from the filter 35 isan aqueous sodium chloride solution contaminated with dissolvedpolyvalent metals. It is conveyed into the column 16 to be treatedtherein as described above with reference to FIG. 1. The solution 17collected from the column 16 is an aqueous solution of sodium chlorideof high purity, substantially free from polyvalent metals and fromsulphate and carbonate ions. It is divided into two fractions, 37 and38.

Fraction 37 is conveyed, in accordance with the invention, into anelectrolysis cell 39 equipped with selectively cation-permeablemembranes. In the electrolysis cell 39 the solution 37 undergoes anelectrolysis in a manner known per se, resulting in the production ofchlorine 40 and of an aqueous sodium hydroxide solution 41.

Fraction 38 is conveyed to an evaporator 42, from which crystallisedsodium chloride 43 and steam 44 are collected. The solid sodium chloride43 finds various industrial applications.

In the plant shown in FIG. 4 a sufficient quantity of hydrochloric acid11 is introduced into the dissolving chamber 9 to acidify the aqueousmixture present therein, to decompose the carbonate ions and to dissolvethe polyvalent metals.

The acidic aqueous medium 12 collected from the reaction chamber 9 is anaqueous solution of sodium chloride and sodium sulphate, additionallycontaining dissolved polyvalent metals. It is conveyed to the filter 13,where the insoluble matter 14 (essentially fly ash) is separated off. Anaqueous solution 45 of sodium chloride and sodium sulphate, containingthe polyvalent metals in the dissolved state is collected from thefilter 13. The solution 45 is conveyed into the reaction chamber 24. Inthe chamber 24 the solution 45 will be alkalified to a pH of between 8and 14 and in addition, the sulphate cations which it contains will beprecipitated in the form of gypsum. For this purpose a milk of lime 46is introduced into the chamber 24 in sufficient quantity to decomposeall of the sodium sulphate in the solution 45 and, if need be, a top-up32 of an aqueous sodium hydroxide solution is also added so as to bringthe pH to the desired value, between 8 and 14. The polyvalent metalsthus coprecipitate in the form of metal hydroxides with calciumsulphate. Consequently an aqueous suspension 33 is collected from thechamber 24 and is treated successively in the settling chamber 34 and onthe filter 35, where a coprecipitate 36 of polyvalent metal hydroxidesand gypsum is separated therefrom and is set aside. An aqueous liquid 15is collected from the filter 35 and is treated as described above withreference to FIG. 3.

In a particular alternative implementation of the embodiments in FIGS. 1to 4, the smoke 2 is treated on an appropriate filter, not shown, toseparate fly ash therefrom and is then introduced into the reactor 3.

In the embodiments described above with reference to FIGS. 1 to 4 thechelating resin in the column 16 must be regenerated at regularintervals. For this purpose the column 16 is isolated from the conduits15 (or 29) and 17 by means of isolating valves 19 and 20, and theoperation is then continued in two successive stages, as described inthe document "The American Institute of Chemical Engineers, No. 219,vol. 78, pages 46-53: J. J. Wolff and R. E. Anderson, Ion-exchangepurification of feed brine for chlor-alkali electrolysis cells; the roleof Duolite ES-467". In a first stage an aqueous hydrochloric acidsolution is introduced into the column 16 via the conduit 21 tosubstitute the protons on the active sites of the chelating resin and adilute aqueous solution of hydrochloric acid containing the polyvalentmetals is collected via the conduit 22. In the second stage an aqueoussodium hydroxide solution is introduced via the conduit 21 in order tosubstitute sodium cations for the protons of the active sites of theresin and a dilute aqueous sodium hydroxide solution is collected viathe conduit 22. The dilute aqueous hydrochloric acid solution and thedilute aqueous sodium hydroxide solution can be recycled into theprocess. For example, in the case of the embodiment of FIG. 4 the diluteaqueous sodium hydroxide solution can be recycled into the reactionchamber 9, and the dilute aqueous sodium hydroxide solution can berecycled into the reaction chamber 24.

What is claimed is:
 1. A process for purification of a smoke containinghydrogen chloride and heavy metals, and for manufacture of an aqueoussolution of sodium chloride, comprising:in a first step, treating asmoke containing hydrogen chloride and heavy metals, with a sodiumbicarbonate powder and reacting the sodium bicarbonate with the hydrogenchloride to form a solid sodium chloride residue with said heavy metals;in a second step, recovering said solid residue containing said sodiumchloride and said heavy metals; in a third step, dispersing said solidresidue from said second step in water to dissolve said sodium chloridethereby forming an aqueous medium including said sodium chloride andsaid heavy metals; in a fourth step alkalinising the aqueous medium ofthe third step to a pH from 8 to 14 to precipitate said metals as heavymetal hydroxides; in a fifth step separating the precipitated heavymetal hydroxides from the aqueous medium of the fourth step andrecovering an aqueous sodium chloride solution; and in a sixth steptreating said sodium chloride solution of the fifth step on a chelatingresin.
 2. The process according to claim 1, wherein said smokeoriginates from the incineration of domestic or municipal waste.
 3. Theprocess according to claim 1, wherein an alkaline aqueous medium is usedto obtain the heavy metals in the form of a precipitate of metalhydroxides, and separating this precipitate from the aqueous liquid on achelating resin.
 4. The process according to claim 3, wherein the pH ofthe aqueous medium is regulated between 8 and
 14. 5. The processaccording to claim 3, wherein a chelating resin containing functionalgroups derived from iminodiacetic and/or aminophosphonic acid is used.6. The process according to claim 3, wherein a chelating resin of the Natype is used.
 7. The process according to claim 3, including acidifyingthe aqueous medium and then filtering, prior to alkalifying the aqueousmedium.
 8. The process according to claim 7, wherein when the aqueousmedium contains SO₄ ²⁻ ions, calcium chloride is added to it when it isacidified, to precipitate calcium sulphate.
 9. The process according toclaim 7, including when said aqueous medium contains SO₄ ²⁻ ions, addingcalcium (hydr)oxide to react with all of the SO₄ ²⁻ ions.
 10. Theprocess according to claim 1, wherein the heavy metals of the smoke areselected from the group consisting of cadmium, mercury, antimony, lead,cobalt, chromium, copper, manganese, vanadium, tin, iron, nickel,aluminum, zinc, and mixtures thereof.
 11. The process according to claim10, wherein the smoke originates from incineration of domestic ormunicipal waste.